An established route to improving the physical properties of a chosen polymer is through the introduction of glass and other fibers, or through other mineral and non-mineral fillers. Generally, improvements may be optimized when good dispersion of the filler and strong interfacial adhesion between the filler and polymer matrix can be achieved. Filling methods may also be cost effective if the filling agents are of moderate cost.
Purely physical mixing of fiber fillers into a polymer have shown some improvements in mechanical properties by the formation of biocomposites. (Mohanty, A. K., Misra, M., Drzal, L. T., Sustainable Bio-Composites from Renewable Resources: Opportunities and Challenges in the Green Materials World. Journal of Polymers and the Environment, 10, No. 1/2: 19-26, Apr. 2002). Biobased fibers including kenaf, hemp, jute, sisal, henequen, pineapple leaf, etc. can be incorporated into degradable, biodegradable, and nondegradable polymers. Flax fibers (about 30-40 wt %) have been embedded into a polylactic acid (PLA) matrix by Oksman et al. (Oksman, K., M. Skrifvars, and J. F. Selin, Natural fibres as reinforcement in polylactic acid (PLA) composites. Composites Science and Technology, 63(9):1317-1324, 2003) who then compared the resulting composite properties to polypropylene (PP) filled with similar fibers. Promising properties of the flax-PLA composites were found; the composite strength was about 50% greater compared to similar flax-PP composites that are industrially employed. However, a lack of interfacial adhesion between the polymer matrix and the fiber surface was suggested by microscopy studies. Improvement of the tensile strength, tensile modulus and impact strength upon reinforcing PLA with cellulose fibers has been observed (Huda, M. S., et al., Effect of processing conditions on the physico-mechanical properties of cellulose fiber reinforced poly(lactic acid). ANTEC 2004 Plastics: Annual Technical Conference, Volume 2: Materials, 2:1614-1618, 2004; Huda, M. S., et al. Physico-mechanical properties of “Green” Composites from poly(lactic acid) and cellulose fibers, at GPEC, Detroit, USA, 2004). However, the introduction of cellulose fibers did not affect the glass transition temperature significantly as measured by DSC. Recently it has been demonstrated that kenaf fibers may be mixed with PLA and if the shortest components of the fiber can be removed and a flexible copolymer is also added, then improvements in impact strength and heat distortion temperatures may be obtained (Serizawa, S., Inoue, K., Iji, M., Kenaf-Fiber-Reinforced Poly(lactic acid) Used for Electronic Products. Journal of Applied Polymer Science, 100: 618-624, 2006). However, such microcomposites are not transparent and also have various challenges associated with processing into useful parts including usually the need to add an anti-microbial agent.
The limitations associated with microcomposites have led to the development of polymer nanocomposites. In such materials, at least one dimension of the filler material is of a size from about 1 to 100 nanometers. Nanocomposites comprised of polylactide as the matrix have been developed (Bhardwaj, R., Mohanty, A. K., Advances in the Properties of Polylactides Based Materials: A Review, Journal of Biobased Materials and Bioenergy, 1: 191-209, 2007). Commonly used nanoscopic fillers (nanofillers) may be clays and other mineral fillers. Clay filled nanocomposites of polylactides have been extensively studied (Ray, S. S., Bousmina, M., Biodegradable polymers and their layered silicate nanocomposites: In greening the 21st century materials world, Progress in Materials Science 50: 962-1079, 2005). Filling of plastics with these ammonium ion containing clays renders them generally unsuitable for food packaging due to toxicity and other considerations; decomposition of the ammonium ions leads to the production of ammonia during processing of the plastics into useful items. Additionally, clays are mined materials which are not renewable. Bioplastic nanocomposites may be based on renewable nanofibers in the same manner that a biocomposite is comprised of a biofiber embedded in a bioplastic. This has led a number of researchers to consider the use of cellulosic nanofibers as reinforcing agents for plastics and bioplastics (Samir MASA, Alloin F, Dufresne A. Review of Recent Research into Cellulosic Whiskers, Their Properties and Their Application in Nanocomposite Field. Biomacromolecules 6: 612-26, 2005). Recent developments using various forms of microcrystalline and nanoscopic cellulose have considerable prior art associated with them.
U.S. Pat. No. 3,109,743 (Rapid Acetylation of Cellulose Crystallite Aggregates) teaches the preparation of cellulosic microcrystallites (microcrystalline cellulose) by hydrolysis of cellulose using about 2.5N hydrochloric acid for about 15 minutes at boiling temperature or in about 0.5% aqueous hydrochloric acid solution at about 121° C. for about 1 hour. Subsequent mechanical disintegration of aggregates can be accomplished and reaction with acetic anhydride in the presence of small amounts of sulfuric acid as catalyst in the absence of acetic acid provides surface modified microcrystalline cellulose. U.S. Pat. No. 4,481,076 (Redispersible Microfibrillated Cellulose) teaches the fabrication and modification of microfibrillated cellulose through repeated passage through a homogenizer followed by the addition of various additives. A suspension which is redispersable after drying may be formed through the use of additives including polyhydroxy compounds, particularly carbohydrates and carbohydrate related compounds, such as cellulose derivatives, glycols, sugars, alcohols, and carbohydrate derivatives, gums, starches, oligo- and polysaccharides. U.S. Pat. No. 4,697,007 (Preparation of microcrystalline triacetylcellulose) teaches acetylation of microcrystalline cellulose with acetic acid and acetic anhydride using perchloric acid as a catalyst. Here, commercially available microcrystalline cellulose may be suspended in a solvent such as benzene, toluene, xylene or a paraffin hydrocarbon and reacted with acetylating agents (acetic acid and/or acetic anhydride) at temperatures up to about 50° C. in the presence of a catalyst. Separation and purification produces a yield of about 80-85% and the product particle size is between about 5 and about 50 μm. U.S. Pat. No. 6,541,627 (Cellulose Dispersion) teaches the formation of stable dispersion in organic solvents through the use of functional additives including polyhydric alcohol, water-soluble polysaccharides, other water-soluble polymers, and other surface active agents. U.S. Pat. No. 6,703,497 (Cellulose microfibrils with modified surface, preparation method and use thereof) teaches surface modification of prepared suspensions by etherification reactions including silylation reactions, etherification reactions, condensation with isocyanates, condensations or substitutions with alkylene oxides, and condensations or substitutions with glycidyl compounds. U.S. Pat. No. 6,967,027 (Microfibrillated and/or microcrystalline dispersion, in particular of cellulose, in an organic solvent) teaches the formation of table suspensions or dispersions of cellulose microfibrils and/or microcrystals in organic solvents without chemical modification of the cellulose. This is accomplished by forming a mixture comprised of an aqueous suspension of cellulose with a surfactant, co-surfactant, and a stabilizing polymer. Despite the many methods proposed for making, isolating, and surface functionalizing cellulosic fillers there is a continuing need for improved performance at lower cost.
Similarly, the use of microcrystalline, microfibrillar, and nanoscopic cellulosics as a filler in polymer composites is well developed. U.S. Pat. No. 6,103,790 (Cellulose Microfibril-Reinforced Polymers and their Applications) teaches the use of very high aspect ratio nanofibers derived from tunican as polymer reinforcement in a variety of plastic matrices. In this art, the glass transition of the polymer matrix is unaffected. U.S. Pat. No. 6,117,545 (Surface Modified Cellulose Microfibrils, Methods for Making the Same, and Use Thereof as a Filler in Composite Materials) teaches the surface modification of previously isolated cellulose nanowhiskers via esterification reactions of the cellulose nanowhisker surface hydroxyl groups with acetic acid or other organic acids. The use of these surface modified nanowhiskers in composite materials is also taught. Recently, it has been demonstrated that cellulose nanowhiskers derived using sulfuric acid and obtained through a multistep process that had been treated with tert-butanol or a surfactant could be well dispersed in a polylactide matrix to create a composite material with superior physical properties (Petersson, L., Kvien, I., Oksman, K., Structure and thermal properties of poly(lactic acid)/cellulose whiskers nanocomposite materials: Composites Science and Technology 67 2535-2544, 2007). Accordingly, while it is known that well-dispersed cellulose nanofillers in polylactide may improve thermal properties, efficient and easily practicable methods for the formation of such composites are lacking.
Thus, there is a desire for improved polymer composites with an improved method of making these filled polymers to achieve the desired polymer physical characteristics rapidly and at an acceptable cost.